Recently, a method of producing a toner (hereinafter also referred to as aggregation toner) by emulsion aggregation has been proposed as a method that can purposely control the surface shape of the toner. In the emulsion aggregation, a toner is usually produced by aggregating fine particles of raw materials having an average particle diameter of 1 μm or less. Therefore, in principle, a toner having a small diameter can be efficiently produced. In addition, a fine textured structure can be easily formed on the surface.
At the same time, recently, saving of energy is a technical issue also in electrophotographic apparatuses, and a large reduction in quantity of heat during fixation of a toner has been being investigated. Accordingly, demands for toners that can be fixed with lower energies, i.e., toners having “low-temperature fixabilities” are increasing.
As a method of enabling fixation of a toner at low temperature, for example, a reduction in glass transition point (hereinafter also referred to as Tg) of a binder resin is performed. However, a reduction in Tg leads to deterioration in heat-resistant storage property of the toner. Therefore, it is difficult to fix a toner at lower temperature.
In order to simultaneously improve the low-temperature fixability and the heat-resistant storage property of a toner, a method of using a crystalline polyester as a binder resin of the toner has been investigated. The crystalline polyester has molecular chains regularly arranged and thereby does not show a defined Tg and has a property not being softened until the melting point. Furthermore, the crystalline polyester sharply melts at the melting point and sharply reduces its viscosity accompanied thereby, that is, the crystalline polyester has a so-called sharp melting property. Accordingly, the crystalline polyester attracts attention as a material that can improve both the low-temperature fixability and the heat-resistant storage property.
PTL 1 proposes a toner produced by a pulverization method using a mixture of a crystalline polyester and a noncrystalline polyester as a binder resin. More specifically, a mixture of a crystalline polyester and a cycloolefin-based copolymer resin is used as a binder resin. However, in this technology, since the ratio of an amorphous material is high, the fixation of the toner tends to be influenced by the Tg of the amorphous material, and therefore the sharp melting property of the crystalline polyester cannot be sufficiently utilized.
Accordingly, technologies of aggregation toners, in which the main component of a binder resin is crystalline polyester and the sharp melting properties thereof are sufficiently exhibited, have been proposed (see PTLs 2, 3, and 4). However, though these toners are excellent in low-temperature fixability, the elasticity at high temperature is insufficient to cause a problem of easily causing high-temperature offset during fixation, and further improvement is demanded. Furthermore, it was revealed that detachment or cracking of toners are caused by printing of a large number of sheets.
In addition, an aggregation toner containing a small amount of a block polymer, in which bound to a crystalline polyester and an amorphous portion are linked to each other, as a binder resin of the toner has been proposed (see PTL 5). In this technology, the fixing property is improved by forming a good dispersion state of the three components: the crystalline polyester, block polymer, and amorphous resin. However, also in this technology, the improvement in durability of a toner in printing of a large number of sheets is restricted, and there is a demand for further improvement.